In power generating plants, especially gas turbines, catalytic reactors, or catalyzers for short, are used to burn a part of the gaseous fuel and air mixture flowing through the catalyzer. Because of this, a temperature increase arises in the gas-air mixture, and depending upon the catalytic reactor a synthesis gas substantially comprising a mixture of hydrogen gas (H2) and carbon monoxide (CO) can also be produced. The hot exhaust gas serves for the thermal and/or chemical stabilization of the homogenous flame in the combustion chamber. An aerodynamic flame stabilization is frequently necessary as, for example, through a sudden cross-sectional enlargement between the catalyzer and the homogenous flame front in the combustion chamber.
The pollutant emission of nitrogen oxides (NOx) and carbon monoxides (CO) can be significantly reduced by the catalytic combustion of fuel-air mixtures. The reason for this reduction is the carbon dioxide (CO2) and water (H2O) present in the exhaust gas of the catalyzer which delay the formation rate of thermally formed nitrogen oxides (NOx) in the homogenous flame front. Therefore, less nitrogen oxide is formed, even at high temperatures above 1450° C. Furthermore, the catalyzers use a thoroughly-mixed fuel-air mixture to avoid a local overheating. As result of this, the homogenous flame mixture is more uniform and local hot spots are avoided which would have promoted the forming of NOx. The direct forming of NOx is also reduced through the smaller hydrocarbon concentrations (CH-concentration) after the catalytic reactor.
The extinction limits for lean flames can also be extended by the chemical stabilization. In particular, hydrogen gas, and up to a certain point also carbon monoxide, have been used for this purpose. With atmospheric burners in gas turbines it was proven that by the substitution of small portions of the gaseous fuel with hydrogen gas the extinction limits could be substantially extended. It is still more advantageous to locally inject the hydrogen gas, whereby less H2 is required than with the premixing with fuel and without the NOx emissions being increased as is the case in the event of poor premixing.
For flame stabilization with catalyzers methods of lean premix combustion are known during which a lean fuel-air mixture is completely oxidized (Full oxidation=FOX) in the catalyzer. With such systems the combustion air and almost all the fuel is routed through the catalyzer. Such systems are prone to fuel-air fluctuations and inhomogeneities and also to a deactivation of the catalyzers. With larger combustion systems a part of the fuel must be bypassed around the catalyzer. The injecting of this fuel after the catalyzer and the admixing can be problematic and can lead to unwanted pollutant emissions.
For flame stabilization with catalyzers methods of rich combustion are also known, in which a rich fuel-air mixture is used. The rich fuel-air mixture is only partially burnt in the catalyzer (Partial oxidation=POX). With these methods, all the fuel is usually directed through the catalyzer. A flame extinction takes place at significantly lower temperatures than with lean mixtures, and the stability and robustness of the catalyzer can be increased considerably. With these systems, however, a large portion of the combustion air can be bypassed around the catalyzer and fed to the exhaust gas after the catalyzer. During this admixing unwanted pollutant emissions and temperature irregularities may occur, especially at high temperatures as are encountered in large combustion systems.